As a means for controlling a speed reduction ratio of a vehicle equipped with a continuously variable speed transmission, there has been known a method in which engine speed is matched with a reference engine speed (which will be hereinafter referred to as R-REF engine speed) set in correspondence to a throttle opening of the engine (see, for example, Japanese Pat. Laid Open Publication No.62(1987)-237164).
Such control may be accomplished by setting a R-REF (running reference) engine speed corresponding to an accelerator opening (i.e., the depressed amount of an accelerator pedal or the throttle opening) when the accelerator pedal has been depressed or the throttle is not closed. However, if the R-REF engine speed is to be set in correspondence to the accelerator opening in the manner as mentioned above, the engine speed will be too low to obtain sufficient engine brake effect when the accelerator pedal is released and the accelerator opening is substantially zero (as encountered in engine braking the vehicle for deceleration).
In order to avoid this problem and to obtain a desirable engine brake, a ratio control has been carried out conventionally by setting a reference engine speed corresponding to the vehicle speed. The reference engine speed is referred to as a EB-REF (engine-brake reference) engine speed hereinafter.
Under such control, if the accelerator opening is reduced to substantially zero during running by releasing the accelerator pedal, a proper EB-REF engine speed is set and a desirable engine brake effect can be obtained. However, the R-REF engine speed thus set in correspondence to the accelerator opening in a region of small accelerator opening becomes lower than said EB-REF engine speed. As a result, when a ratio control is switched from an engine brake mode where the accelerator opening is substantially zero (the control based on the EB-REF engine speed) to a running mode where the R-REF engine speed is set based on the accelerator opening, the R-REF engine speed may become very low. Namely the R-REF engine speed may become lower than the EB-REF engine speed if the transition from the engine-brake mode to the running mode is made in a region of small accelerator opening. Accordingly, a small depression of the accelerator pedal under the engine brake mode will lower the engine speed, giving the driver an uncomfortable driving feeling.